A hydraulic shock absorber for a vehicle comprises, for example, a piston delimiting two oil chambers in a cylinder, an oil passage penetrating the piston to connect the two oil chambers, and a damping valve. The damping valve may be constituted by a laminated leaf valve provided in an outlet of the oil passage.
When the piston strokes in the cylinder, the respective oil chambers expand and contract and working oil flows through the oil passage. Accompanying this action, the damping valve generates a damping force attributed to a flow resistance that the damping valve exerts on the working oil, thereby damping a shock or oscillation input into the shock absorber.
Since the laminated leaf valve remains in a closed state until a predefined opening pressure is achieved, the damping force generated when a piston stroke speed is in a low piston stroke speed region may become excessively large. Considering the riding comfort of the vehicle, it is preferable to cause the generated damping force to vary in a linear fashion with respect to the piston stroke speed especially in the low piston stroke speed region.
With respect to damping force characteristics of a shock absorber in a low piston speed region, JP2001-165224 A, published by the Japan Patent Office in 2001, proposes laminating sub-leaf valves having notches and holes on a leaf valve. JP2007-132389 A, published by the Japan Patent Office in 2007, proposes small diameter chokes penetrating a piston so as to bypass the laminated leaf valve, thereby improving the damping force characteristics in the low stroke speed region.